The ATMega328 used on the Uno uses A4 & A5 for I2C, so for the Uno they're the same pins.

On the Leonardo, however, the ATMega32U4 uses pins D2 and D3 for I2C. The Due also uses something other than A4/A5.

If a shield needs I2C connectivity and uses A4/A5 it will only work with a Uno (and the older 328/168 boards). If a shield uses the SCL/SDA pins, however, it should work with all the Arduinos (except the old ones, which don't have those pins)